On gas trimmers, both two-stroke internal combustion engines (ICE) and four-stroke are used. But the ignition system in them is no different from each other, so the repair and adjustment of this fuel ignition system will be the same for different types of ICE.
How the trimmer ignition system works
The ignition system of a two-stroke ICE consists of 2 elements: a flywheel and a coil (magneto). The image below shows how they look.
On the flywheel is an impeller that acts as a fan to cool the engine. Also on one side of the flywheel are permanent magnets, shown in the figure by an arrow.
When the flywheel rotates, an electric current is generated in the coil, which serves to produce a spark on the spark plug. A spark appears when the magnets are directly below the magneto.
Why do I need to adjust the ignition
In some cases, when the engine does not start, ignition adjustment will be required. Of course, first you need to check the serviceability of the candle and high-voltage cable with a cap.
Ignition control on a two-stroke engine is necessary in order to ensure timely ignition of fuel in the engine combustion chamber. Below is a diagram from which you can understand the principle of the internal combustion engine.
A spark in the candle should appear at the moment of compression of the fuel by the piston, when it does not reach the top dead center (TDC) just a little. When the piston passes TDC, the fuel mixture ignites, as a result of which the piston goes down under the action of the explosion energy.
Therefore, if for some reason (mainly due to the displacement of the flywheel on the shaft relative to its initial position), the fuel ignites before the piston passes TDC, then it goes back and the crankshaft scrolls in the opposite direction. This movement can damage the starter assembly, cut off the starter cord, etc. In this case occurs early fuel ignition.
May also be late ignition on the trimmer: after the compression of the fuel, which did not ignite, the piston goes down, and at that moment a spark appears. In this case, the engine will either not start, or it will significantly lose power and will not gain momentum well.
But such problems with early or late ignition on two-stroke ICEs are quite rare, since the flywheel with magnets is already installed in the correct position, which is perfectly synchronized with the movement of the piston. In addition, the flywheel is fixed to the shaft with a dowel and nut. Therefore, incorrect installation of the part is excluded.
Basically, the ignition adjustment is to set the correct clearance between the magneto and the flywheel.
Video: Ignition Coil Adjustment On Stihl Trimmer
How to check the health of magneto
Magneto is the main part in the engine ignition system and is a coil with primary and secondary windings located around the core. Often, due to a magneto malfunction, a spark disappears on the spark plug, and ignition of the fuel does not occur. To test the coil, you will need to use a tester, and perform the following steps.
- On the tester, going into resistance measurement mode, you need to set 200 Ohms. In this way, the primary winding will be checked.
- Connect one electrode of the tester to the “mass” of the coil, and the second to the contact coming out of the magneto. Normally, the resistance should be between 0.4 and 2 ohms.
- To check the secondary winding, you will need to set the switch of the device to 20 com. Further, one probe of the device is inserted into the cap of the candle, and the second is connected to the “mass”. Normally, the resistance indicators should be in the range from 4-6 kom and above (depending on the model).
When the circuit breaks, the device will show infinity, and when closed. 0 (zero).
If you don’t have a tester, then you can use the following method, with great care so that the electrode does not touch the cylinder, otherwise the coil will burn out.
- Remove the cap from the candle. You do not need to twist the candle to maintain compression.
- Insert a nail without a cap into the cap. The nail should be of such a diameter that it holds well in the cap.
- Tie the cap with the nail to the cylinder using a dielectric so that the gap between the electrode and the cylinder body is between 5.5 and 7 mm (but in no case touches the “mass”).
- Using a starter, simulate the engine starting several times. The ignition button must be on.
- Observe whether the spark appears and what color it has.
Normally, the spark should be strong, white or have a bluish tint. If the spark is weak, and its color is orange or yellow. this indicates a faulty magneto.
It should also be borne in mind that sometimes a cold magneto can produce a good spark. But after heating due to internal malfunctions, it ceases to produce current. As soon as the coil cools down, the trimmer starts again and works for a while until the magneto is again heated. Such a coil must be replaced.
Sometimes a spark plug can cause a similar heating problem, even if the coil is working, so both parts need to be checked.
Ignition on the trimmer is fairly easy to adjust.
Remove the casing from the engine by unscrewing all the bolts holding it. For example, on the Patriot (Patriot) PT 2540 lawn mower, to remove the casing, you will need to unscrew the air filter housing and remove it from the latches with a flat screwdriver. After that, the plastic case can be easily removed.